1. Technical Field
The present invention generally relates to detector elements for focal plane arrays, and more particularly, to a microbolometer detector element having a discrete thermally isolating structure for yielding high fill factor.
2. Discussion
Microminiature bolometers (microbolometers) such as those described in U.S. Pat. No. 5,602,393, entitled "MICROBOLOMETER DETECTOR ELEMENT WITH ENHANCED SENSITIVITY", issued Feb. 11, 1997, to Henry M. Gerard and in U.S. Pat. No. 5,286,976, entitled "MICROSTRUCTURE DESIGNED FOR HIGH IR SENSITIVITY", issued Feb. 15, 1994, to Barrett E. Cole, are used as detector pixel elements in two-dimensional IR imaging arrays. Two-dimensional detector arrays are commonly used in optical sensors to convert an observed scene into an electronic image for processing and displaying.
A microbolometer generally consists of a polycrystalline semiconductor whose electrical resistivity varies as a function of its temperature. The semiconductor layer material is chosen so that it absorbs optical radiation over a design wavelength range, which is generally in the IR region of the spectrum. The semiconductor layer is fabricated on a silicon substrate, which also contains integrated readout circuitry for monitoring the layer's resistivity. An array of microbolometers may be fabricated on a single substrate to create a two-dimensional imaging array.
In operation, incident IR radiation is absorbed by the semiconductor layer, causing a change in the layer's temperature. The temperature change causes a corresponding change in the layer's resistivity, which is monitored by the readout circuitry. The ultimate signal-to-noise ratio of the microbolometer is a function of sensor thermal mass and thermal isolation from supporting structure.
Related art microbolometers utilize a continuous semiconductor absorptive layer deposited on a dielectric "bridge" structure that has been fabricated on the silicon substrate. The bridge structure supports the layer so that it is spaced away from the silicon substrate surface. To accomplish this, conventional, uncooled microbolometers use a single sacrificial layer between the semiconductor layer and the silicon substrate during manufacturing. When the sacrificial layer is removed, the isolation support structure and the semiconductor absorptive layer are located in the same plane.
However, the current trend in microbolometer technology is towards larger formats and smaller pixels. When the pitch of the pixel is reduced for use in larger formats and for higher spacial resolution, a finite support leg length is still required in order to maintain adequate thermal isolation for responsivity performance. As such, the percentage of area that the support structure occupies increases as the pixel size decreases. This results in the absorptive area becoming smaller and smaller which reduces the fill factor and sensitivity and leads to a corresponding decrease in performance.
Therefore, it would be desirable to provide a microbolometer having a higher ratio of active detector area to total area of the pixel unit cell comprising the active detector area (fill factor) than according to the prior art. More particularly, it would be desirable to provide a microbolometer having a discrete, independently controllable, thermal isolation structure for supporting the semiconductor absorptive layer while providing an enhanced fill factor. Additionally, it would be desirable to provide a microbolometer having a thermal isolation structure which also serves as a reflective layer for reflecting incident optical radiation not absorbed by the semiconductor absorptive layer back to the semiconductor absorptive layer.